U.S. patent number 4,456,208 [Application Number 06/435,511] was granted by the patent office on 1984-06-26 for shell tile thermal protection system.
This patent grant is currently assigned to The United States of America as represented by the Administrator of the. Invention is credited to H. Neale Kelly, Ashby G. Lawson, Ian O. MacConochie.
United States Patent |
4,456,208 |
MacConochie , et
al. |
June 26, 1984 |
Shell tile thermal protection system
Abstract
A reusable, externally applied thermal protection system is
disclosed and functions by utilizing shell tile structure 10 which
effectively separates its primary functions as an insulator and
load absorber. Tile 10 consists of structurally strong upper and
lower metallic shells 12,16 manufactured from materials meeting the
thermal and structural requirements incident to tile 10 placement
on the spacecraft. A lightweight, high temperature package of
insulation 26 is utilized in upper shell 12, while a lightweight,
low temperature insulation 28 is utilized in lower shell 16.
Assembly of tile 10, which is facilitated by self-locking mechanism
20, may occur subsequent to installation of lower shell 16 on the
spacecraft structural skin 30.
Inventors: |
MacConochie; Ian O. (Yorktown,
VA), Lawson; Ashby G. (Tabb, VA), Kelly; H. Neale
(Yorktown, VA) |
Assignee: |
The United States of America as
represented by the Administrator of the (Washington,
DC)
|
Family
ID: |
23728699 |
Appl.
No.: |
06/435,511 |
Filed: |
October 20, 1982 |
Current U.S.
Class: |
244/159.1;
244/117A; 428/76 |
Current CPC
Class: |
B64G
1/58 (20130101); Y10T 428/239 (20150115) |
Current International
Class: |
B64G
1/22 (20060101); B64G 1/58 (20060101); B64G
001/58 () |
Field of
Search: |
;244/158A,160,163,117A,121 ;220/306,445
;52/404,405,506,513,805,809 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Blix; Trygve M.
Assistant Examiner: Corl; Rodney
Attorney, Agent or Firm: Osborn; Howard J. Manning; John R.
Nelson; Wallace J.
Government Interests
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United
States Government and may be manufactured and used by or for the
Government for governmental purposes without the payment of any
royalties thereon or therefor.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. A shell tile adapted for use as a component part of a reusable,
externally applied thermal protection system on the skin of
aerospace vehicles and the like comprising:
upper and lower metallic shells for assuming structural loads,
each of said upper and said lower shell having a base and side
portions,
said metallic shells being constructed from materials able to
withstand temperatures of up to approximately 2300.degree. F., said
upper shell base being exposed directly to the high temperature
environment during vehicle flight;
at least one dimpled, lightweight metallic sheet being attached
interiorly within said upper shell adjacent the base thereof,
thereby adding stiffness and strength to said upper shell;
each of said upper and said lower shell having a base and side
portions,
said lower shell base being attached to the vehicle skin and said
upper shell being disposed opposite and joined at the side portion
thereof to said lower shell, defining an interior cavity
therebetween;
insulating means interposed in said interior cavity; and
said insulation means including at least one layer of lightweight
high temperature insulation inserted into said upper shell and at
least one layer of lightweight intermediate temperature insulation
inserted into said lower shell.
2. A shell tile adapted for use as a component part of a reusable,
externally applied thermal protection system on the skin of
aerospace vehicles and the like comprising:
upper and lower metallic shells for assuming structural loads,
each of said upper and said lower shell having a base and side
portions,
said lower shell base being attached to the vehicle skin and said
upper shell being disposed opposite and joined at the side portion
thereof to said lower shell, defining an interior cavity
therebetween;
insulating means interposed in said interior cavity;
said insulation means including at least one layer of lightweight
high temperature insulation inserted into said upper shell and at
least one layer of lightweight intermediate temperature insulation
inserted into said lower shell;
said high temperature insulation comprising a 98.5+% pure silica
fiber with a density of 3.5 lb/cu ft; and
said intermediate temperature insulation comprising a 98.5+% pure
silica fiber with a density of 1.1 lb/cu ft.
3. A shell tile adapted for use as a component part of a reusable,
externally applied thermal protection system on the skin of
aerospace vehicles and the like comprising:
upper and lower metallic shells for assuming structural loads,
each of said upper and said lower shell having a base and multiple
side portions,
said lower shell base being attached to the vehicle skin and said
upper shell being disposed opposite and joined at the side portion
thereof to said lower shell, defining an interior cavity
therebetween;
insulating means interposed in said interior cavity;
said lower shell being telescopically received by said upper shell
and including locking means for joining the shell side portions in
telescoped connection;
said locking means utilizing a self-locking mechanism including at
least one upper tapered wedge being attached to the outer surface
of at least two sides of said upper shell,
at least one lower tapered wedge being attached to the outer
surface of an equal number of sides of said lower shell,
said upper shell wedges having a flat upper surface and a tapered
edge pointing downwards and said lower wedges having a flat lower
surface and a tapered edge pointing upwards;
said upper and said lower wedges being disposed opposite one
another so as to allow said upper wedge to traverse over said lower
wedge as downward pressure is applied to said upper shell causing
said upper shell sides to spread, until said upper wedge is
completely beyond said lower wedge, thereby permitting the flat
surfaces thereof to contact for prohibiting reverse movement of
said shells due to the inherent spring force exerted thereon by
said upper shell sides;
restraining means for prohibiting further downward movement of said
upper shell once said tapered wedges are locked in place;
said restraining means comprising supporting wedges disposed on the
outer surface of said sides of said lower shell, thereby impeding
further downward movement of said upper shell.
4. A shell tile adapted for use as a component part of a reusable,
externally applied thermal protection system on the skin of
aerospace vehicles and the like comprising:
upper and lower metallic shells for assuming structural loads,
each of said upper and said lower shell having a base and side
portions,
said lower shell base being attached to the vehicle skin and said
upper shell being disposed opposite and joined at the side portion
thereof to said lower shell, defining an interior cavity
therebetween;
insulating means interposed in said interior cavity;
connecting means for attaching said lower shell base to the vehicle
skin while permitting some relative movement therebetween;
said connecting means including a floating nut plate integral with
the vehicle skin, and bolting means integral with said lower shell
for directly connecting said lower shell to the vehicle skin.
5. A structure as in claim 4 wherein said floating nut plate
comprises:
nut means for securing said bolting means in place;
retention means for encasing said nut means, said retention means
being securely fastened to the inner surface of said skin and
constructed so as to permit limited lateral and vertical movement
of said nut means, while preventing rotational movement
thereof;
said retention means also having an apertured portion in its upper
surface and a lower surface comprising a retaining lip thereby
allowing said bolting means to penetrate and connect to said nut
means; and
said bolting means being of such diameter as to form a loose fit
through said apertured portion and said skin, thereby reducing the
strain between said skin and said tile structure by allowing
limited lateral movement of the nut-bolt combination during periods
of thermal or mechanical expansion.
Description
BACKGROUND OF THE INVENTION
This invention relates to a reusable, externally applied thermal
protection system for use on aerospace vehicles subject to high
thermal and mechanical stresses, and more particularly to a shell
tile structure which effectively separates its primary functions as
an insulator and load absorber.
Because space vehicles are subject to temperature extremes during
ascent and re-entry, it is customary to provide the vehicle with a
heat shield designed to protect the vehicle metallic substructure.
The advent of the Space Shuttle initiated the need for a reusable,
nonablative thermal protection system (TPS). The orbiter vehicle
basically utilizes a conventional, skin-stringer aluminum aircraft
structure. The properties of aluminum, however, dictate that the
maximum operating temperature of the substructure not exceed
350.degree. F. In addition, the vehicle is subjected to multiple
aerodynamic loads during flight, including aerodynamic pressure
gradients and shocks, buffet and gust loads, acoustic pressure
loads caused by boundary layer noise and concomittant substructure
motion. Therefore, the TPS used must protect the aluminum
substructure from high surface temperatures and at the same time
withstand the thermal cycles and environmental loads of space
flight.
The ceramic tile utilized on the initial flights of the shuttle,
however, has relatively low strength and a low coefficient of
thermal expansion as compared to metals. The relatively low tile
strength precludes use for load bearing applications and dictates
that the tiles be secured to the protected structure by a means
which will minimize transfer of strains from the metal structure to
the tile. Because of its homogeneous structure and brittle
character, the ceramic tile has a strain to fracture performance
considerably below the yield strain of aluminum and, as a result,
must be monitored carefully for surface erosion, fraying and
cracking. In addition, its low coefficient of thermal expansion is
a deterrent to the tile ability to protect against gap heating.
It is preferable, therefore, to design a TPS which effectively
separates its functions as an insulator and load absorber. This
separation of functions allows flexibility in designing a TPS which
can withstand thermal and mechanical stresses more effectively,
without adding to the weight of the vehicle.
Accordingly, it is an object of this invention to provide an
improved thermal protection system for aerospace vehicles.
Another object of this invention is to provide a reusable,
externally applied thermal protection system which effectively
separates its functions as an insulator and load absorber.
Another object of this invention is to provide a durable thermal
protection system which utilizes a structurally strong outer shell
to assume loads and a lightweight, thermally efficient insulation
internally to resist heat flow.
Another object of this invention is to provide a thermal protection
system which substantially reduces gap heating of the aerospace
vehicle structure.
Still another object of this invention is to provide a thermal
protection system which can be readily altered to meet desired
thermal and mechanical performance.
Yet another object of this invention is to provide a thermal
protection system which is strong in shear and tension, thereby
permitting it to be directly connected to the substructure.
Still another object of this invention is to provide a thermal
protection system which has a low parts count and is easily
manufactured, installed, replaced and repaired.
SUMMARY OF THE INVENTION
The foregoing and other objects of the invention are achieved by
providing a reusable, externally applied thermal protection system
on an aerospace vehicle comprising a shell tile structure which
effectively separates its primary functions as an insulator and
load absorber. The system utilizes a plurality of tiles, each of
which consists of structurally strong upper and lower metallic
shells manufactured from materials meeting the thermal and
structural requirements incident to tile placement on the
spacecraft. A lightweight, high temperature package of flexible
insulation is utilized in the upper/outer shell, while a
lightweight, low temperature flexible insulation is utilized in the
lower/inner shell. Assembly of the tile, which is facilitated by
self-locking wedges on both shells, may occur subsequent to
installation of the lower shell on the spacecraft structural
skin.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the
attendant advantages thereof will be readily apparent as the same
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings wherein:
FIG. 1 is a view of an aerospace vehicle utilizing the tile thermal
protection system of the present invention;
FIG. 2 is a cross-sectional view of an individual assembled
shell-tile structure of the present invention utilized by the
aerospace vehicle of FIG. 1 and taken along line II--II of FIG.
4;
FIG. 3 is an exploded bottom view of the components of the shell
tile structure of FIG. 2 prior to assembly;
FIG. 4 is a schematic front view of an assembled tile illustrating
the relative size and location of the self-locking mechanism
utilized in the present invention;
FIG. 5 is an enlarged part-sectional view of the self-locking
mechanism taken along line V--V of FIG. 4;
FIG. 6 is a part-sectional view of the floating nut plate
connection utilized in one embodiment of the present invention;
FIG. 6a is a cross-sectional view of the adhesive connection
utilized in another embodiment of the present invention; and
FIG. 7 is a graphic illustration of the response of the present
invention to artificially induced conditions of extreme temperature
at the specific points on the tile as designated in FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, FIG. 1 illustrates an exemplary
aerospace vehicle, generally designated by reference numeral 5,
that utilizes a thermal protection system 9 made up of multiple
tiles 10 of the present invention. Because vehicle 5 is subject to
extreme temperatures during re-entry into the Earth atmosphere as
well as during other hypervelocity flight, it requires thermal
protection in varying degrees throughout. Consequently, the area on
the nose cap and leading edges of the wings must withstand the
effects of temperatures in excess of 2300.degree. F. Most of the
lower surface of vehicle 5 must withstand temperatures in the range
of 1200.degree. F. to 2300.degree. F. The side surface of the tail,
and the forward top and sides of vehicle 5 must withstand
temperatures in the range of 700.degree. F. to 1200.degree. F.
Finally, the upper surface of the wings and the top and rearward
sides of vehicle 5 must withstand temperatures up to 900.degree.
F.
Referring now more particularly to FIGS. 2 and 3, there is shown an
individual shell tile structure 10 of the present invention. Tile
10 consists primarily of a structurally strong upper shell or cap
12 and a lower shell or casing 16 which utilize high temperature
insulation 26 and intermediate temperature insulation 28,
respectively, in their interior cavities and are connected by
self-locking mechanism 20 (FIGS. 4 and 5).
Upper and lower metallic shells 12, 16 function primarily to assume
the aerodynamic loads to which the spacecraft is subject during
flight. Basically rectangular in shape, shells 12, 16 are disposed
opposite one another so as to define an interior cavity between
them. Upper shell 12 is slightly larger than and fits over lower
shell 16, completely enclosing the interior cavity. Also, in the
preferred embodiment, most of the bottom of lower shell 16 is
removed, leaving only retaining lip 18 to facilitate connection to
the vehicle substructure 30. This decreases the overall weight of
tile 10, which may be fabricated into any external shape required
for the particular area of the spacecraft to be insulated.
Because tile 10 is exposed directly to the external environment,
upper and lower shells 12, 16 must be manufactured from materials
which meet the thermal and structural requirements incident to tile
10 placement on the spacecraft. Candidates for upper shell 12
include titanium and titanium alloys which typically retain their
structural characteristics up to approximately 750.degree. F.;
nickel alloys, such as Rene-41, which are typically effective up to
approximately 1600.degree. F. (Renee-41 is commercially available
from General Electric and other suppliers); columbium, effective to
approximately 2200.degree. F.; and reinforced carbon composites,
effective to approximately 3000.degree. F. Candidates for lower
shell 16 include titanium, because of its low weight and
conductivity, and graphite/polyimide composites, among others. In a
specific example tile with shells 12 and 16 formed of Rene-41, a
material thickness of 0.004" proved adequate. Other materials would
be expected to be within the thickness range of 0.004 to 0.100. The
latter thickness applying to low density organic composites.
Another objective of utilizing structurally strong metallic shells
12, 16 is to reduce gap heating between tiles 10. As a metal or any
other material is subject to increasing temperatures, it will
expand as a function of its coefficient of linear thermal
expansion. Because of its low coefficient of linear thermal
expansion, the ceramic tile presently used on the Space Shuttle
lags considerably behind the expansion of the aluminum substructure
30 when exposed to extreme temperatures. Therefore, gaps occur
between the tiles, and the substructure 30 is exposed to the high
temperature environment. Tile 10 of the present invention is
designed to reduce this effect substantially. By utilizing
materials with a coefficient of linear thermal expansion similar to
that of the aircraft substructure 30, tile 10 will expand
coextensively with substructure 30 during periods of high thermal
stress. This expansion will effectively close the gaps and reduce
direct heating of substructure 30.
To achieve this result, materials must be chosen and tile 10 placed
such that upper shells 12 of adjoining tiles 10 are closest to each
other at the moment of maximum shell 12 expansion. The following
calculations illustrate the method of determining the proper gap at
installation.
The general expansion of any material is defined as
where,
.alpha.=coefficient of linear thermal expansion
L=length of the material sample
.DELTA.T=temperature differential (T.sub.final -T.sub.initial).
Therefore, the size of the gap resulting from coextensive expansion
of tile 10 and substructure 30 is defined as
Because tile 10 will expand before substructure 30, the calculation
must be approached in two stages. First, the initial expansion of
tile 10 must be determined. This calculation represents what
typically occurs at the beginning of entry. In this specific
example, upper shell 12 is constructed of Rene 41 and substructure
30 of aluminum: ##EQU1## Therefore, shell 12 will differentially
expand 0.0324 inch at the beginning of entry and should be so
spaced at installation. By installing at this interval, shells 12
of adjoining tiles 10 will touch upon maximum expansion. (This
calculation presupposes that when upper shell 12 reaches
900.degree. F., substructure 30 is approximately 110.degree. F. Of
course, this is a function of the type of entry trajectory and
insulation 26,28 utilized, and the location on the vehicle.)
By the end of flight, however, aluminum substructure 30 has had a
chance to heat up and expand, thereby pulling tiles 10 away from
each other and causing a gap therebetween. This counter expansion
assumes that substructure 30 heats to 250.degree. F., and is
calculated as follows: ##EQU2## Therefore, if the gap at
installation is 0.0324 in., the gap at the end of entry will be
##EQU3## By utilizing materials with different coefficients of
linear thermal expansion, and changing tile 10 placement, this gap
can be altered accordingly and the effect of gap heating
substantially reduced.
Within shells 12,16 are placed several layers of lightweight,
thermally efficient insulation. Typically, upper shell 12 is filled
with one or more types of high temperature insulation 26 which has
high density and remains effective at very high temperatures. One
example is Micro-Quartz.RTM., manufactured by the Johns-Manville
Aerospace Company. This 98.5+% silica fibrous insulation has a
density of 3.5 lb/cu ft and a thermal conductivity of 0.83
Btu-in/sq ft-hr-.degree.F. at 1000.degree. F. Lower shell 16, on
the other hand, utilizes insulation 28 which is lighter, less dense
and remains effective only to certain intermediate temperatures.
One example is a 98.5+% silica fibrous insulation with a density of
1.1 lb/cu ft and a thermal conductivity of 0.48 at 600.degree.
F.
Because upper and lower shells 12, 16 bear the structural loads,
the choice of insulation 26, 28 is dependent only on the
temperature profile between the outer surface of upper shell 12 and
substructure 30 sought to be achieved through tile 10 use.
Therefore, lightweight but thermally efficient insulation may be
utilized. In addition, each cap may use several layers of
insulation 26,28, each with different properties, once again
depending only on the desired thermal performance of tile 10. In
determining the desired thermal characteristics of any particular
tile 10, reference must be made to the exact location of tile 10,
as the degree of protection required will change with relative
position on vehicle 5.
Referring to FIGS. 4 and 5, the self-locking mechanism 20 is more
clearly illustrated. Mechanism 20 functions to hold upper shell 12
in place once tile 10 is assembled. Basically, it utilizes tapered
wedges 32,34 disposed, respectively, on each side of upper and
lower shells 12,16. Upper wedges 34 are disposed on the inner
surfaces of the sides to upper shell 12 with their tapered edge
pointing downwards, while lower wedges 32 are disposed on the outer
surface of the sides to lower shell 16 with their tapered edge
pointing upwards. Wedges 32,34 are disposed opposite one another so
as to allow upper wedge 34 to traverse over lower wedge 32 as
downward pressure is applied to upper shell 12. Once upper wedge 34
is completely beyond lower wedge 32, shells 12,16 are locked in
place and no reverse movement can occur. The material forming shell
12 has inherent spring action physical property characteristics
permitting pressure expansion thereof and spring retraction and
retention once the wedges 34 move beyond lower wedges 32. To
prohibit further downward movement of upper shell 12, mechanism 20
utilizes supporting wedges 36 disposed on the outer surface of
lower shell 16. Although the number of wedges may be varied, in the
preferred embodiment a pair of wedges 32,34 was employed on each
shell side. One support wedge 36 was employed with each pair of
lock wedges. These wedge numbers could vary with for example, only
two pair of lock wedges 32, 34 being provided (one pair on each of
opposite sides) with four support wedges 36 still employed (one on
each shell side). On a tile 10 having six inch square sides, wedges
32, 34 and 36 of one-quarter inch width by one-quarter inch long,
by one-sixteenth inch thick, proved adequate.
Mechanism 20 permits easy assembly and maintenance of tile 10.
Because upper shell 12 can be readily removed (via the spring
action of the side portions thereof), the type of insulation 26,28
utilized may be changed to accommodate any anticipated change of
conditions. Therefore, the operational characteristics of tile 10
may be readily altered by replacing the insulation 26,28 with
alternate types and thicknesses of insulation and/or by changing
the material and guages of upper and lower shells 12,16. To further
enhance the insulating qualities of a tile additional
non-structural loose sheets of dimpled and undimpled foils of
approximately 0.001 inch thick of high emissivity metals can be
added to act as radiation barriers above insulation 26 and below
upper shell 12. High temperature tapes can also be bonded around
the perimeter of lower shell 16 in contact area between upper and
lower shells 12,16 to further enhance insulating qualities of the
tile. None of the above changes require alteration of the thickness
of the tile, the outer moldline of the vehicle, or basic design of
the tile. In addition, this design permits assembly of tile 10 to
take place subsequent to disposition of lower shell 16 on
substructure 30. Once lower shell 16 is attached, insulation 26,28
may be inserted and upper shell 12 attached by applying downward
pressure until self-locking mechanism 20 operates. Conversely, any
individual tile can be removed by first forcing thin wedges between
the tiles until the wedge rests between lower shell 16 to release
self-locking mechanism 20.
Because shells 12, 16 are strong in shear and tension, there is no
need to isolate tile 10 from the strain of aluminum substructure
30. Therefore, lower shell 16 may be attached directly to
substructure 30. One means of facilitating installation is the
floating nut plate apparatus 22 shown in FIG. 6. Apparatus 22
utilizes a retention plate 46 which is riveted, welded or otherwise
securely fastened to the inner surface of substructure 30. Within
plate 46 is nut 44 which is free to move laterally. Because of the
shape of plate 46, i.e., flat surfaces abutting flat edges of nut
40, nut 44 may not move rotationally. Bolt 40 is placed through
retention lip 18 of lower shell 16 and attached to nut 44 through
bore 42, lower shell 16 being then secured in place. Apparatus 22
is typically utilized in each corner of lower shell 16. Because
bore 42 is slightly larger in diameter than bolt 40, apparatus 22
may move laterally during periods of thermal or mechanical
expansion. By permitting lateral movement, apparatus 22 can
effectively accommodate any thermal expansion experienced by lower
shell 16 without causing additional strain between lower shell 16
and substructure 30.
Another means by which lower shell 16 may be attached to
substructure 30 is illustrated in FIG. 6a wherein an elastomeric
room-temperature-vulcanizing adhesive (RTV) 50 is substituted for
lock apparatus 22. Adhesive 50 retains its adhesive characteristics
up to approximately 450.degree. F.-500.degree. F. and may be used
either as the sole connecting means or in conjunction with
apparatus 22. Specific example adhesives meeting this criteria are
General Electric's RTV-560 and Hysol Company's EA-934 Epoxy.
Although not required, a strain isolation pad (SIP) 24 was employed
in the preferred embodiment and interposed between lower shell 16
and substructure 30 prior to installation of either of the
above-mentioned connecting means. Strain isolation pad 24 reduces
any residual interstructural strain caused by expansion of
substructure 30 and effectively hinders the flow of heat from the
bottom of lower shell 16 to substructure 30. Pad 24 may comprise,
for example, one or more layers of Nomex felt. A tradename of the
DuPont Company, Nomex is a poly(1,3-phenylene-isophtalamide).
FIG. 7 is a graphic illustration of the response of a typical tile
10 to artificially induced conditions of extreme temperatures. Tile
10 utilized to gather this data is that shown in FIG. 2. Upper
shell 12 is constructed of 0.004 inch thick Rene 41 and has a
square outer surface measuring 6 inches.times.6 inches and sides
measuring 1.65 inch. In the preferred configuration a dimpled sheet
14 of 0.004 inch Rene 41 is electron beam welded to and within
upper shell 12 and serves to reduce radiation heat transfer and to
add stiffness and strength to the outer surface of upper shell 12.
High temperature insulation 26 utilized in upper shell 12 is
Micro-Quartz.RTM. , manufactured by Johns-Manville Aerospace
Company. This insulation 26 comprises 98.5+% pure silica fibers and
has a density of 3.5 lb/cu ft. Lower shell 16 is constructed of
0.004 inch thick titanium and has sides measuring 1.5 inch and
retaining lip 18 measuring 0.75 inch. Lower shell 16 also utilizes
an upper retainer 19 measuring 0.187 inch which adds strength to
lower shell 16. A strain isolation pad 24 comprising a layer of 0.2
inch thick Nomex felt is interposed between lower shell 16 and
substructure 30. Connection of lower shell 16 to substructure 30 is
facilitated by floating nut plate apparatus 22. The low temperature
insulation 28 utilized comprises 98.5+% pure silica fibers with a
density of 1.1 lbs/cu ft.
The curves in FIG. 7 correspond to the labelled points in FIG. 2
and illustrate the temperature at that point as a function of time.
Therefore, curve A is a plot of temperature vs time for point A,
curve B is a plot for point B, etc. Initially, tile 10 is subjected
to steadily increasing temperatures until upper shell 12 reaches
900.degree. F. (Section I). This temperature is then maintained for
500 seconds while the effect on the entire tile structure 10 is
monitored (Section II). The temperature is then allowed to drop off
(Section III). These results indicate approximately a 300.degree.
F. drop in temperature through the upper layer of insulation 26,
and a further drop of approximately 400.degree. F. through low
temperature insulation 28. At no time does the temperature of the
aircraft substructure 30 exceed approximately 110.degree. F. By
changing the insulation 26,28 utilized in the interior cavity of
tile 10, this temperature profile may be altered accordingly. In
addition, by utilizing high temperature glass insulation tape
between shells 12,16 at their interface, the flow of heat to
substructure 30 may be hindered further.
Although the invention has been described relative to a specific
application thereof, it is not so limited and numerous variations
and modifications will be readily apparent to those skilled in the
art in light of the above teachings. It is therefore to be
understood that within the scope of the appended claims, the
invention may be practiced otherwise than as specifically
described.
* * * * *